Mi skribas notojn sur reuzpapero.

Because it contains two endings:

• -j = plural, so notoj = notes
• -n = accusative, marking the direct object

In this sentence, the notes are what the speaker is writing, so notojn is the direct object of skribas.

Because reuzpapero is not the direct object here. It is part of the prepositional phrase sur reuzpapero.

After a preposition like sur, Esperanto normally uses the basic form of the noun. You add -n after a preposition only when showing direction or movement toward something. Here the sentence describes where the writing is, not motion onto the paper.

Because when you write on paper, you write on its surface. Esperanto expresses that with sur = on.

Using en would mean in or inside, which does not fit normal writing on paper. So sur reuzpapero is the natural choice.

In Esperanto, the present tense -as covers both:

• I write
• I am writing

So Mi skribas can describe either a general action or something happening right now. Context tells you which meaning is intended.

Esperanto has no indefinite article, so there is no separate word for a or an.

It does have a definite article, la, for the, but you use it only when the noun is specific or already known. In Mi skribas notojn sur reuzpapero, the nouns are being used in a general or indefinite way, so no article is needed.

Esperanto very often forms compound words by joining roots together. The final part is the main idea, so reuzpapero is a kind of papero connected with reuz-.

This is a very normal feature of Esperanto word-building. English often uses separate words where Esperanto prefers a single compound.

Here papero is being used like a material or type of thing, not as a countable set of individual papers. Esperanto often uses a singular noun in that kind of situation.

So sur reuzpapero means something like writing on reuse-related paper as a material or medium, not necessarily on one single sheet only.

Yes. Esperanto word order is fairly flexible because endings show the grammatical roles.

Mi skribas notojn sur reuzpapero is the most neutral order. But for emphasis, you could move parts around, for example Sur reuzpapero mi skribas notojn. The basic meaning stays the same.

Because Esperanto does not copy every English idiom directly. The natural expression is skribi notojn, literally write notes.

This is very common in language learning: even when the meaning is the same, a different language may choose a different everyday verb.

Yes. With prepositions of place like sur, Esperanto can use -n to show direction.

So:

• sur reuzpapero = on reused/reuse paper, location
• sur reuzpaperon = onto reused/reuse paper, direction

In your sentence, location is what matters, so sur reuzpapero is the correct form.